This invention relates to a thermal recording device and a structure of a thick film type thermal head mounted thereon, and in particular, to a thermal head suitable for full-color high-quality images.
As disclosed in JP-A (Laid open, hereinafter, called as A)-55-84683 and JP-A-57-89980, when the protective layer of a thermal head is imparted with high thermal conductivity, it is conventional practice to form an abrasion-resistant layer on a protective layer surface of a thin film thermal head by sputtering or evaporating a substance having high hardness and high thermal conductivity thereon. The abrasion-resistant layer is intended to give an improved heat-transfer coefficient between the thermal head and a coloring medium.
A thick film type thermal head uses, in general, an electrode system of an alternate lead structure, and therefore has the following characteristic defect. When an image is printed, no current flows in a resistor positioned directly on the electrode. That is, that portion of the resistor which is positioned directly on the electrode generates no heat. Hence, a low-temperature area occurs on the thermal head surface, and a white streak appears on the printed image in a subscanning direction. Further, JP-A-1-128849 describes that the thermal efficiency is improved by incorporating a metal oxide into a protective glass of a thick film type thermal head.
A conventional thick film type thermal head uses, in general, an electrode system having an alternate lead structure. The use of this system has the following problem. When an image is printed, no current flows in that portion of the resistor which is positioned directly on the electrode as shown in FIG. 16, and isothermic lines on the resistor surface are therefore distributed concentrically about the central portion of a pixel as shown in, the Figure. A non-heat developing portion results in a low-temperature area and produces a white streak on the printed image in a subscanning direction. The subscanning direction means the direction in which a coloring medium is fed, and the main scanning direction means the direction which is at an right angle with the subscanning direction, i.e. the direction in which the heating resistors are arranged. An arrow mark in FIG. 16 shows the main scanning direction.
In prior techniques disclosed in JP-A-55-84683, JP-A-57-89978 and JP-A-57-89980, a high thermal conductivity layer having abrasion resistance is formed in order to improve thermal contact between a thermal head and a coloring medium. JP-A-1-128849 discloses incorporation of a metal oxide into a protective glass in order to improve heat efficiency, whereby the thermal conductivity of a protective layer is increased. However, it fails to take the following into consideration: nonuniformity in density caused by a temperature distribution difference on the heating resistor surface and abrasion between the thermal head surface and the coloring medium or resolution of a printed image.